1. Field of the Invention
The present invention relates to a light-emitting device, an exposure device, and an image forming apparatus. More particularly, the present invention relates to an exposure device which serves as a light source for exposing a photoconductive member in an apparatus utilizing the electrophotographic process, and an image forming apparatus employing the exposure device.
2. Description of the Related Art
The electrophotographic process is, as is well known, a process comprising the steps of exposing an image onto a photoconductive member, developing the image with a toner, transferring a toner image onto a transfer material (paper), fusing a transferred image, and then cleaning the photoconductive member. The electrophotographic process is utilized in many image forming apparatuses.
In such an image forming apparatus, a laser optical system for scanning a laser beam with a polygon mirror is widely employed as an exposure unit for writing a latent image onto the surface of a photoconductive member because the laser optical system has a high resolution and a high scanning speed. However, the laser optical system has problems in that difficulties are encountered in reducing the unit size because a space to accommodate optical parts such as a polygon mirror and lenses is required, and in realizing a superhigh speed because a laser beam is dynamically scanned with rotation of the polygon mirror.
For that reason, attention has been focused on an LED (light-emitting diode) array in which a plurality of LEDs formed of GaAsP, GaAlAs or the like are set into an array. The LED array has been increasingly applied to exposure units for use with the electrophotographic process because the LED array is a linear light-emitting unit and performs static scanning with a simpler optical system.
Further, another linear light-emitting array has been proposed in which array elements are formed at a time using organic or inorganic light-emitting elements. An array of light-emitting elements is collectively formed, for example, by stacking a transparent anode layer, an organic compound layer, and a cathode layer on a long belt-shaped light permeable base plate such that the belt-shaped organic compound layer is held between both the electrode layers having predetermined patterns from the front and the back, thus enabling the individual light-emitting elements to emit light independently of one another.
However, the light-emitting element array, and in particular, the light-emitting element array employing organic light-emitting elements, has a problem in that the amounts of light emitted from the light-emitting elements are reduced due to temperature changes depending on successive light emissions.
In the LED array using a plurality of LEDs and the linear light-emitting element array in which array elements are formed at a time using organic or inorganic light-emitting elements, because a number of light-emitting elements are arranged, the light-emitting elements of the array have individual differences in characteristics and do not have the same light-amount characteristics. Thus, there has been a problem that even with the light-emitting elements driven at the same level, light emission outputs of the elements slightly differ from one another and the amounts of light emitted from the elements vary to some extent.
As a consequence, when applied to exposure units for use with the electrophotographic process, variations in the amounts of light emitted from the light-emitting elements give rise to unevenness in exposure and hence unevenness in density of the formed image, whereby image quality is deteriorated.
In view of the above problems experienced in the related art, an object of the present invention is to provide a light-emitting device, an exposure device, and an image forming apparatus which can compensate for and changes in the amount of light attributable to temperature changes depending on successive light emissions.
According to a first aspect, the present invention provides a light-emitting device comprising a light-emitting element array formed by arranging a plurality of light-emitting elements, driving means for driving the light-emitting element array to emit light from each of the light-emitting elements, memory means for storing the number of light emissions for each light-emitting element of the light-emitting element array, and control means for controlling the driving means based on the information stored in the memory means so that the amount of light emitted from each light-emitting element is held constant. According to the first aspect, the present invention also provides an exposure device employing the light-emitting device, and an image forming apparatus employing the exposure device.
According to a second aspect, the present invention provides a light-emitting device comprising a light-emitting element array formed by arranging a plurality of light-emitting elements, driving means for driving the light-emitting element array to emit light from each of the light-emitting elements, memory means for storing a total light-emitting time for each light-emitting element of the light-emitting element array, and control means for controlling the driving means based on the information stored in the memory means so that the amount of light emitted from each light-emitting element is held constant. According to the second aspect, the present invention also provides an exposure device employing the light-emitting device, and an image forming apparatus employing the exposure device.